Emergence of a Multiple Input Multiple Output (MIMO) technology brings about revolutionary changes to wireless communication. By deploying multiple antennas on both a transmit end device and a receive end device, the MIMO technology can provide multiple mutually independent channels simultaneously. Therefore, a data transmission rate is doubled.
FIG. 1 is a schematic diagram of an application scenario of MIMO. The application scenario shown in FIG. 1 includes a transmit end device 102 and a receive end device 104. The transmit end device 102 may be, for example, a base station, but is not limited thereto. The receive end device 104 may be, for example, a terminal device, but is not limited thereto. n(n>1) transmit antennas are configured for the transmit end device 102, and are specifically denoted as transmit antennas 1 to n. m(m>1) receive antennas are configured for the receive end device 104, and are specifically denoted as receive antennas 1 to m. In this way, a total of m×n channels exist between the n transmit antennas and the m receive antennas, as shown by solid lines between the transmit antennas 1 to n and the receive antennas 1 to m (some channels are not shown).
In an ideal situation (for example, without considering noise), the m×n channels may be indicated by using the following channel matrix:
                    H        =                  [                                                                      h                  11                                                            …                                                              h                                      1                    ⁢                    n                                                                                                      ⋮                                            ⋱                                            ⋮                                                                                      h                                      m                    ⁢                                                                                  ⁢                    1                                                                              …                                                              h                  mn                                                              ]                                    (        1        )            
where hij(1≤i≤m, 1≤j≤n) indicates a channel gain of a channel between a transmit antenna j and a receive antenna i. The receive end device 104 may determine the channel matrix by using a pilot (Pilot) transmitted by the transmit end device 102.
As shown in FIG. 2, the m×n channels may be equivalent to i channels that are mutually independent. The channels are also referred to as beams (beam), and may be obtained by using multiple technologies, for example, a precoding (Precoding) technology. The precoding technology defines a beam by using a precoding matrix (Precoding Matrix). Specifically, each column vector also referred to as a precoding vector (Precoding Vector) of the precoding matrix corresponds to one beam. An element of the column vector corresponds an antenna or antenna port (Antenna Port)) on a one-to-one basis, and is used for weighting an antenna, so that signals transmitted by the antenna overlap each other to form a beam. A maximum quantity of beams of the terminal device is equal to a quantity of singular values of the channel matrix, that is, a rank (Rank) of the channel matrix. When some singular values are excessively small (lower than a preset threshold), a quantity of the singular values that are excessively small is subtracted from the foregoing quantity. For ease of description in the specification, the rank of the channel matrix is uniformly used to indicate the quantity of the channels that are mutually independent.
The precoding technology uses a preset codebook to record the precoding matrix. The receive end device 104 selects, based on the channel matrix and based on a preset selection criterion for example, including but not limited to a maximum channel capacity criterion, a minimum mean square error criterion, or a minimum singular value criterion, an appropriate codeword in the codebook to indicate the precoding matrix. In each selection criterion, there are multiple algorithms for selection.
The existing codebook is designed for a Line Of Sight (LOS) channel, but actually most channels are Non Line of Sight (NLOS) channels. Therefore, when a precoding matrix of an NLOS channel is determined based on a LOS codebook, precision is quite limited.